Chemical, enzymatic and microbial degradation of bacterial and synthetic poly-β-hydroxyalkanoates

Abstract

The chemical, enzymatic and microbial degradation of bacterial and synthetic poly-β-hydroxybutyrate (PHB) and its copolymers with β-hydroxyvalerate (PHB/HV) were evaluated. Alkaline hydrolysis (7·4 and 10 pH units) was carried out at 70°C. Samples were treated with purified extracellular PHB depolymerase from Alcaligenes faecalis T1. Microbial degradation was studied using a chemostat with an inoculum of activated sludge under carbon limited conditions. Samples studied were synthetic PHB, bacterial PHB and PHB/HV. Samples were in the form of dry powders, a ‘never dried’ aqueous dispersion (latex) of native granules or films made from these materials. The microbial degradation of PHB/HV latex coated paper samples was also studied. Bacterial PHA samples degraded completely after 12 h exposure to PHB depolymerase. Degradation in a microbial environment was within a matter of days. Conversely, synthetic PHB samples which degraded more readily than bacterial PHB by alkaline hydrolysis did not degrade as readily upon exposure to a microbial environment. The extent of degradation was determined quantitatively by weight loss and was followed qualitatively by scanning electron microscopy. The degradation rate depended on sample crystallinity, tacticity, morphology and composition. In general PHB/HV films and coated paper made with the ‘never dried’ latex degraded more quickly than solvent cast films. Synthetic PHB of intermediate tacticity exposed to depolymerase enzyme biodegraded more extensively than highly isotactic synthetic PHB.